Is necessary to actually have an address in the country of registration?

i.e. resident in Austria, yacht in Croatia & registration in the UK.

I shall be sailing mainly in the Mediterranean in croatian waters.

To register a boat in the UK, one must have a physical contactable address in the UK. Once registered the boat you may fly the Red Ensign and cruise Croation waters.

As far as Austria is concerned - contact with the following link may provide the information regarding registering the boat in Austria - Many Austrian boats are designed and built in Austria for cruising the Danube and beyond :-CLICK